One aspect of the present invention is directed to an insulation layer used in plastic molding applications. Recent improvements in the molding of plastic parts include providing a multilayer composite mold structure in which a thermally conductive core or a cavity, e.g., a metal core or cavity, is covered with an insulating layer, generally of a polymer, over which a hard skin metal layer is provided for improving the surface characteristics of a molded article made from such a mold structure. The purpose of the insulating layer is to slow the cooling of the thermoplastic material sufficiently so that heat from the plastic material remains within the polymer melt for a period of time sufficient to prevent formation of voids, die lines, folds and other surface defects created during the solidification of the plastic melt. Otherwise, the plastic material which comes in contact with the relatively cool surface of the mold core, quickly solidifies and the surface defects are frozen in place on the molded article surface. Such an insulating layer is sometimes also known as a thermally engineered multilayered insulated structure (TEMIS) mold surface.
The present invention recognizes a problem that exists in injection molding processes employing thermal insulation layers of the type discussed above. In particular, where a metal skin is employed over the insulation layer to improve the surface characteristics of a finished article, delamination may occur at the interface of the metal skin and the underlying insulation layer, especially near areas adjacent to the gate through which molten polymer is injected into the mold cavity. The present invention is directed to improving the adhesion of the metal skin to the underlying insulation layer.
Another aspect of the present invention is directed to metallized polymer substrates used in various industries. Ever since structural plastics have replaced metal in the enclosures used in electrical appliances, microwave ovens, business machines, and other electrical/electronic products, manufacturers have had to overcome problems caused by electromagnetic interference (EMI) in general and radio frequency interference (RFI) in particular. The Federal Communications Commission (FCC), since 1983, requires that the electrical products not exceed certain specified EMI/RFI levels. The FCC requirements have been codified in the FCC regulation CRF 47, Part 15, Subsection J. The FCC requirements are met by reducing the EMI/RFI emission from the electrical/electronic products by providing a shielding. With the increased sensitivity of newer, higher-speed, and higher-frequency circuits plus a continued proliferation of electronic devices worldwide, EMI shielding problems are becoming more demanding. This has placed greater emphasis on high signal attenuation by the shielding medium.
The EMI shielded enclosures are also used to protect delicate electronic/electrical circuitry and components enclosed within the enclosure from damage by external sources such as static electricity or man-made high intensity EMI emissions.
Enclosures having metal cases, metal foil claddings, wire mesh screens, applied coatings, magnetic materials, and a variety of alternative approaches have been tried. However, because of their cost advantages and superior performance, plastic enclosures having metallized coatings have emerged as the dominant choice.
Several attempts have been made to increase the adhesion of a conductive metal layer to halogenated polyimide substrates. One of the prior art methods for improving adhesion involves grit blasting the surface to provide a roughened profile on which the subsequently-applied metals can be anchored. Other methods call for the use of chemical swelling agents or penetrants to swell the surface prior to the application of a metal layer.
While such methods do increase adhesion, they are often not entirely satisfactory for several reasons. Such techniques often result in physical degradation of the halogenated polyimide surface thereby decreasing the tensile as well the impact strength of the underlying halogenated polyimide substrate. The aforementioned physical degradation results from the swelling and cracking steps to which the entire substrate material is exposed. Additionally, such surface preparations can cause crack formation and propagation at highly stressed areas such as at sharp corners or edges of the enclosure being shielded.